The inclination dependence of gold tracer diffusion along a Σ3 twin grain boundary in copper

Grain boundary impurity diffusion of Au in diffusion bonded Σ3〈011〉 Cu bicrystals has been studied by the radiotracer serial-sectioning technique using the ¹⁹⁵Au isotope with high specific activity. The dependence of the grain boundary diffusion parameter sδD_b on the inclination angle Φ of the grain boundary plane with respect to the close-packed (111) plane of the coherent twin boundary has been determined for the temperatures of 673 and 773 K (here s, δ and D_b are the segregation factor, the grain boundary width and the grain boundary diffusion coefficient, respectively). At both temperatures sδD_b exhibits a minimum at Φ=70.5° and a maximum at Φ=82.0°. It is shown that there is no direct correlation between the energy and diffusivity of the grain boundaries in this system and that the periodicity in the grain boundary plane has a significant influence on the grain boundary diffusivity.     

Temperature dependence of faceting in Σ5(310)[001] grain boundary of SrTiO3

Using high-resolution electron microscopy (HREM), temperature dependence of faceting of an asymmetric Σ5 grain boundary (GB) in SrTiO₃ is observed. The bicrystal samples have been fabricated by ultra-high vacuum diffusion bonding and heat-treated between 1100 and 1600 °C. Below 1300 °C, this GB facets into symmetric (310) and asymmetric (100)//(430) GB planes. At 1300 °C, in addition to the asymmetric facet, the symmetric {210} facet appears: three different facets are thus observed at this temperature. At 1400 and 1500 °C, the asymmetric facet disappears and the two kinds of symmetric facets remain. At 1600 °C, faceting disappears and the GB becomes defaceted. The faceting/defaceting transition behavior of the GB is interpreted in terms of the Wulff plot and its corresponding equilibrium crystal shape.     

Atomic structure of the Σ5 (310)/[001] symmetric tilt grain boundary in molybdenum

Atomistic simulations offer an important route towards understanding and modeling materials behavior. Incorporating the essential physics into the models of interatomic interactions is increasingly difficult as materials with more complex electronic structures than f.c.c. transition metals are addressed. For b.c.c. metals, interatomic potentials have been developed that incorporate angularly dependent interactions to accommodate the physics of partially filled d-bands. A good test of these new models is to predict the structure of crystal defects and compare them with experimentally observed defect structures. To that end, the Σ5 (310)/[001] symmetric tilt grain boundary in Mo has been fabricated and characterized by HREM. The experimentally observed structure is found to agree with predictions based on atomistic simulations using angular-force interatomic potentials developed from model generalized pseudopotential theory (MGPT), but disagrees with predictions based on radial-force potentials, such as those obtained from the Finnis–Sinclair method or the embedded atom method (EAM).     

Aluminum Σ3 grain boundary sliding enhanced by vacancy diffusion

Grain boundary sliding is an important deformation mechanism for elevated temperature forming processes. Molecular dynamics simulations are used to investigate the effect of vacancies in the grain boundary vicinity on the sliding of Al bi-crystals at 750 K. The threshold stress for grain boundary sliding was computed for a variety of grain boundaries with different structures and energies. These structures included one symmetrical tilt grain boundary and five asymmetrical tilt grain boundaries. Without vacancies, low energy Σ3 grain boundaries exhibited significantly less sliding than other high energy grain boundaries. The addition of vacancies to Σ3 grain boundaries decreased the threshold stress for grain boundary sliding by increasing the grain boundary diffusivity. A higher concentration of vacancies enhanced this effect. The influence of vacancies on grain boundary diffusivity and grain boundary sliding was negligible for high energy grain boundaries, due to the already high atom mobility in these boundaries.     

Tracer diffusion of Au and Cu in a series of near Σ=5 (310)[001] symmetrical Cu tilt grain boundaries

Grain boundary diffusion of Au and Cu was measured in a series of Cu bicrystals with symmetrical near Σ=5, Θ=36.9° (310)[001] CSL tilt grain boundaries (GBs) using the radiotracer and the serial sectioning technique. The orientations of the bicrystals were very precisely determined with the Kossel technique where all three macroscopic parameters describing the orientations of the grains in the bicrystal were evaluated. The tilt angles ranged from 33.21° to 39.26°. The GB diffusion of the radiotracers ¹⁹⁵Au and ⁶⁴Cu was measured as a function of tilt angle and temperature. In the investigated temperature range 1030–661 K the orientation dependence of both radiotracers shows a characteristic cusp not exactly at but slightly below the ideal Σ=5 CSL GB. The Arrhenius parameters, activation enthalpy and frequency factor, determined from lower temperature data adopt a maximum, again slightly before the ideal Σ=5 CSL GB. These features are discussed with respect to the accidental small twist and second tilt orientations and the corresponding dislocation network inherent in the investigated real GBs. With increasing temperature a negative deviation from a straight Arrhenius behaviour is observed. This result indicates a certain change in the GB structure in the temperature range above 800 K.     

Silicon Σ13(5 0 1) grain boundary interface structure determined by bicrystal Bragg rod X-ray scattering

The atomic structure of the silicon Σ13(5 0 1) symmetric tilt grain boundary interface has been determined using Bragg rod X-ray scattering. In contrast to conventional structural studies of grain boundary structure using transmission electron microscopy, this approach allows the non-destructive measurement of macroscopic samples. The interface was found to have a single structure that is fully fourfold coordinated. X-ray diffraction data were measured at Beamline I07 at the Diamond Light Source.     

Characteristic features of diffusion induced grain boundary migration for Σ9 [110] asymmetric tilt boundaries in the Cu(Zn) system

The characteristic features of diffusion induced grain boundary migration (DIGM) in the Cu(Zn) system were experimentally studied for [110] asymmetric tilt boundaries using Cu bicrystals annealed at 693 K for various times by a capsule zincification technique. The experiment was carried out for the boundaries with inclination angles of φ=0, 20, 35, 55, 65 and 90° and with a constant misorientation angle of θ=39° (Σ9). During annealing, the grain boundary migrates towards a crystal grain of larger coherency strain energy with higher probability due to DIGM. Taking elastic anisotropy of each crystal grain into consideration, the difference between the probabilities of grain boundary migration towards both side grains can be accounted for by the coherency strain model proposed by Hillert. The migration rate v of the moving boundary is almost constant regardless of the annealing time t between t=72 and 384 h (2.59×10⁵ and 1.38×10⁶ s). The value of v monotonically decreases with increasing inclination angle. This implies that the boundary diffusion coefficient of Zn in Cu is a monotone decreasing function of the inclination angle. The experimental results on the kinetics at the steady state stage were theoretically analyzed using the energy balance model proposed by Kajihara and Gust (Scripta mater., 1998, 38, 1621. The analysis indicates that the effective driving force Δ^efG for the grain boundary migration is merely one-thirtieth of the chemical driving force, whereas it still remains three times greater than the minimum value corresponding to the coherency strain energy during DIGM under the present experimental conditions. The mobility of the moving boundary was evaluated to be M=3.77×10⁻¹⁷ m⁴/Js from the values of v and Δ^efG according to the relationship M=v/Δ^efG. This value of M is close to the results estimated by Yamamoto et al. (Acta mater., 1999, 47, 1757) for the [100] twist and random boundaries in the Cu(Zn) system.     

Insight into gallium behavior in aluminum grain boundaries from calculation on Σ=11 (113) boundary

Gallium impurities affect the atomic processes and material properties of aluminum metal to a high degree. Various ab initio calculations have been performed on a Σ=11 (113) symmetric tilt boundary in aluminum with and without some gallium substitutions. A simple interpretation of the results emerges, which can be applied to grain boundaries in general. The calculations relate to the energetics of gallium substitution on various sites, local relaxation effects, vibrational frequencies and a barrier to grain boundary migration.     

In situ investigation of liquid Ga penetration in Al bicrystal grain boundaries: grain boundary wetting or liquid metal embrittlement?

The phenomenon of grain boundary penetration (GBP) of liquid Ga along grain boundaries (GB) of Al bicrystals is investigated by synchrotron radiation X-ray microradiography. From the three different types of bicrystals studied, only the one with the highest GB energy showed GBP in the absence of applied external stress. In situ observations of the penetration process reveal a linear propagation of the penetration front, accompanied by a continuous thickening of the wedge-shaped Ga layer in the GB. The experimental results demonstrate that GBP kinetics are strongly influenced by very weak levels of stress and tend to indicate that such stresses may be a prerequisite for the formation of nanometric penetration layers.     

The mechanism of coercivity enhancement by the grain boundary diffusion process of Nd–Fe–B sintered magnets

We discuss the mechanism of the coercivity enhancement by the grain boundary diffusion process (GBDP) using Dy vapor based on detailed microstructural characterizations. Scanning electron microscopy and electron probe microanalysis showed that a (Nd,Dy)₂Fe₁₄B shell formed in the outer region of Nd₂Fe₁₄B grains while its thickness decreased from the surface to the center of a cube-shaped sample. Atom probe tomography showed that the Dy content at grain boundaries (GBs) was close to that in the (Nd,Dy)₂Fe₁₄B shell. High-temperature annealing (at 900 °C) of a GB diffusion processed magnet led to the disappearance of the GB layers, which resulted in a substantial reduction in coercivity. This suggests that both the (Nd,Dy)₂Fe₁₄B shell and the Nd-rich GB phase layer are required microstructural features for the coercivity enhancement by the GBDP.     

Faceting behavior of an asymmetric SrTiO3 Σ5 [001] tilt grain boundary close to its defaceting transition

Using high-resolution transmission electron microscopy (HRTEM), an asymmetric Σ5 [001] grain boundary in SrTiO₃ with an inclination of 2.5° away from the symmetric (310) plane was studied. A defaceting transition is observed between 1500 and 1600 °C. From 1520 to 1570 °C, the grain boundary is faceted into the symmetric (310) and asymmetric (100)//(430) planes. At 1590 °C, the grain boundary is observed to be defaceted, which implies that the defaceting occurs between 1570 and 1590 °C. The faceted structure observed between 1500 and 1590 °C is interpreted in terms of the step free energy of the asymmetric (100)//(430) facet orientation. The faceting-defaceting transition is schematically represented as the temperature evolution of the equilibrium grain shape.     

Formation mechanism of texture during dynamic recrystallization in γ-TiAl,nickel and copper examined by microstructure observation and grain boundary analysis based on local orientation measurements

Texture formation in γ-TiAl, nickel and copper during dynamic recrystallization (DRX) in compression deformation is studied. The formation process is discussed in relation to the mechanism of new grain formation during DRX. It is found that the behavior of texture formation varies depending on Z (the value of the Zener-Hollomon parameter) and the kind of material. During deformation in low Z conditions, sharp fiber texture develops in γ-TiAl, suggesting that strain-induced grain boundary migration is predominant in the new grain formation. No sharp texture is seen in nickel and copper after the dynamic recrystallization in low Z conditions. However, microstructure observation suggests that new grains are also generated by strain-induced grain boundary migration in these two pure metals. It is found that the difference of texture formation between γ-TiAl and the other two pure metals is ascribed to the difference in the frequency of twinning during grain boundary migration; texture develops only when the frequency is low, which is the case of γ-TiAl.     

Interphase boundary fracture and grain boundary precipitation of Ni3Al(γ′) phase in β-NiAl bicrystals

The effect of the crystallography of film-like Ni₃Al(γ′) precipitates along grain boundaries of NiAl(β) on the fracture stress at room temperature was examined using β bicrystals with controlled orientations. The selected variant of γ′-film satisfied the Kurdjumov–Sachs (K–S) relation with a neighbouring β crystal, and deviated from the relation with an adjacent β crystal. In the course of tensile deformation at ambient temperature, fracture occurred preferentially at the (β/γ′) interphase boundary deviating from the K–S relation, which showed no plastic flow, and the fracture stress decreased with increasing deviation angle. In contrast, slip transfer from γ′-film to β crystal across coherent (β/γ′) interface was observed, when the variant of γ′-film satisfied the K–S relation with both neighbouring β crystals. To clarify the relation between the interphase boundary fracture and the deviation angle from the K–S relation, the boundary structure of incoherent (β/γ′) interfaces was discussed using O-lattice theory and transmission electron microscopic observations.     

Kinetic equation based on non-steady-state diffusion of oxygen in solid copper

The kinetics of internal oxidation of dilute Cu-Al alloys, containing 0. 447 5%- 2. 214%Al (mole fraction) was investigated over the temperature range of 1 023 - 1 273 K and the depth of internal oxidation was measured by microscopy. Based on non-steady-state diffusion, a rate equation is derived to describe the kinetics of internal oxidation of plate: X=k√t, where X is the oxidation depth, t is the oxidation time. For the internal oxidation of Cu-Al alloys employed in the synthesis of alumina dispersion strengthened copper, the permeability of oxygen in solid copper is obtained from the internal oxidation measurements. Investigation shows that the depth of the internal oxidation is a parabolic function of time, the typical shape of the front of internal oxidation is of planar morphology,and there is no evidence for preferential diffusion along grain boundaries.     

Survey of computed grain boundary properties in face-centered cubic metals—II: Grain boundary mobility

The absolute grain boundary mobility of 388 nickel grain boundaries was calculated using a synthetic driving force molecular dynamics method; complete results appear in the Supplementary materials. Over 25% of the boundaries, including most of the non-Σ3 highest mobility boundaries, moved by a coupled shear mechanism. The range of non-shearing boundary mobilities is from 40 to 400 m/s GPa, except for Σ3 incoherent twins which have mobilities of 200–2000 m/s GPa. Some boundaries, including all the 〈1 1 1〉 twist boundaries, are immobile within the resolution of the simulation. Boundary mobility is not correlated with scalar parameters such as disorientation angle, Σ value, excess volume or boundary energy. Boundaries less than 15° from each other in five-dimensional crystallographic space tend to have similar mobilities. Some boundaries move via a non-activated motion mechanism, which greatly increases low-temperature mobility. Thermal roughening of grain boundaries is widely observed, with estimated roughening temperatures substantially among boundaries.     

Electrochemical behaviour of copper–silver alloys in sodium carbonate aqueous solution

Abstract

The electrochemical behaviour of Cu–20 wt-%Ag and Cu–80 wt-%Ag alloys was studied in aerated Na₂ CO₃ solutions using cyclic voltammetry, potentiodynamic anodic polarisation, and current transient techniques. The microstructure of the compounds formed on the surface of the alloy during the anodic potential sweep was obtained using XRD analysis. The dissolution behaviour of either of the two constituents from the alloys resembles that of the pure state. The galvanic coupling effect enhances the dissolution of the less noble metal, copper, on alloying with silver. The anodic sweep potential exhibited seven anodic peaks A₁ , A₂ , A₃ , A₄ , A₅ , A₆ , and A₇ prior to the oxygen evolution reaction. These peaks are assigned to the formation of Cu₂O, Cu(OH)₂ , CuO, Ag₂O, Ag₂ CO₃ , and Ag₂O₂ respectively. Potentiostatic current/time transients showed that the formation of Cu₂O, CuO, Ag₂O, and Ag₂ CO₃ layers involves a nucleation and growth mechanism under diffusion control.